An offset dual-reflector aerial combined with at least one reflector is well known. It also is well known that, in the dual-reflector aerial such as Cassegrain aerials and Gregorian aerials, the so-called reflector surface shaping method is applied to the main reflector and the subreflector, these constituting an aerial, in order to improve electric characteristics such as gain, side lobe level, noise temperature, etc.
The original analysis on the off-set aerial has been described by J. S. Cook, E. M. Elam, and H. Zucker in an article "The Open Cassegrain Antenna Part I. Electromagnetic Design and Analysis" BSTJ, Vol. 44, No. 7, Sept. 1965, pages 1255 to 1300 and the reflector surface shaping method has been described by Victor Gallindo, IEEE, "Design of Dual-Reflector Antennae with Arbitrary Phase and Amplitude Distributions" IEE Trans. on Ap. WL. Ap-12, No.-4, July 1964, pages 403 to 408. Other literature describing the same is an article "Effect of Subreflector Radiation Pattern on the Radiation Characteristic of a Cassegrain Antenna Subjected to Reflector-Surface Shaping" described by Motoo Mizusawa in Nihon Denshi Tsushin Gakkai-shi (Journal of the Electronic and Communication Society of Japan, 1969)
When such a method is applied to a large-diameter aerial, it is preferable, from an economical viewpoint, that the main reflector be constructed from a large number of congruent reflector-segments. FIG. 1 shows a segmentation of one of the conventional, rotational symmetrical type Cassegrain aerials reflector-surface shaped of rotational symmetrical type.
In FIG. 1, rings P.sub.1 to P.sub.n are comprised of a number of congruent reflector-segments (a.sub.1, b.sub.1,-m.sub.1) . . . (a.sub.n, b.sub.n,-m.sub.n). The center cross-sectional figure of each ring P.sub.1 to P.sub.n depends on only the radial segmentation and not on the peripheral segmentation. When such reflector segments are molded, n curved molds are necessary but the same curved mold may be used irrespective of the peripheral segmentation. This is advantageous in that the necessary kinds of the curved molds are reduced, thus leading to the reduction of the manufacturing cost and the stabilization of the production accuracy.
When an offset dual-reflector aerial as shown in FIG. 2 is constructed by using the reflector-segments, two segmentations as shown in FIGS. 3(a) and (b) are imaginable. In the case of the segmentation shown in FIG. 3(a), the reflector-segments are all different in shape so that this type segmentation is disadvantageous, from an economical viewpoint. In the case of the segmentation in FIG. 3(b), when the reflector system belongs to a revolutional secondary curved-surface system (a.sub.i, b.sub.i,-n.sub.i), the center cross section of each reflector segment is identical and therefore this type segmentation is advantageous in the cost reduction of its manufacturing.
When the conventional reflector surface shaping method is applied to the main- and sub-reflector system, in order to improve the electric characteristic of the offset dual-reflector aerial of the revolutional secondary curved-surface system, the main reflector does not constitute a part of a secondary curved-surface of rotational symmetry.
Curves shown in FIG. 4 represent a typical cross-sectional figure of the main reflector whose surface is shaped. In the figure, the curve designated by 12 represents a cross section taken on the line X--X' in FIG. 3(b) and the curve by 13, a cross section taken along the line Y--Y'. When such a segmental reflector is molded by curved molds, for example, the respective segments are not congruent. Therefore, such a case needs curved molds equal in number to the reflector-segments, with the result that it is not economical, the production accuracy is poor and thus the electric characteristic is deteriorated.